During integrated circuit (IC) chip fabrication and packaging, IC chips are tested and burned in at high power to determine whether they perform as required. The high power testing exposes the IC chip to high temperatures. As IC chip feature sizes have continually decreased, more power is required for driving a unit area of silicon (Si) devices, resulting in increased heat generation. In addition, higher burn-in powers result in higher heat gradients across the IC chip.
A typical burn-in testing tool comprises a test socket and heat sink as well as related mechanical structures and electronics. During burn-in testing, the tester heat sink is thermally coupled to the IC chip to dissipate heat.
Heat sinks are also widely used in electronics, particularly in integrated circuits such as microprocessors, digital signal processors, graphic processing units, and other components that require a heat sink to reduce their temperatures through increased thermal mass and heat dissipation.
Previous generations of IC chips were cooled using heat sinks that were coated with a flat, soft alloy to improve thermal contact. Liquid thermal interfaces (LTIs) have also been placed between the heat sink and the IC chip to compensate for non-planarities in the chip surface and the mating heat sink surface. LTIs function to minimize the occurrence of any air gaps between the two surfaces, which would result in a significant reduction in thermal performance and the ability to remove heat from the chip. Poly-alpha olefin (PAO) oil has been used as an LTI to increase heat transfer. However, PAO oils can degrade at the high temperatures and long times necessary for burn-in testing. Water based fluids have also been used as LTIs in test. Despite excellent thermal performance, they have not proven suitable for use at high temperatures, and can cause corrosion of sensitive devices.
The use of LTIs as a cooling solution further poses a risk of corrosion of the controlled collapse chip connection (C4) as a result of LTI entering the C4 cage. The C4 is an interconnect mounting technology for semiconductor devices developed by International Business Machines Corp. The use of LTIs as a cooling solution also requires additional cleaning steps after test if the material does not evaporate fully.
Another current cooling solution involves the use of a dry thermal interface, in which air or helium is injected into the interface between the IC chip and the heat sink. The use of this cooling solution requires that the testing capabilities be reduced in order to avoid overheating the chip, thus making it inappropriate for future generations of IC chips with smaller features.
Because IC chips have irregular surfaces, there are usually only a few points of contact between the heat sink and the chip when the heat sink is pressed into contact with the IC chip with a force. The remainder of the area will have a gap which is defined by the flatness of the two surfaces and the force applied. The larger the gap between the heat sink and the IC chip, the less efficient the cooling means will be.